Jeez I guess it's a rampant problem with those raggidy built Sentra's. I bought mine and left the Nissan dealer without checking the sound system...big mistake. The girl that had it before me (loaner) blew the front speakers out, with the rears not as hot either.
It would help if you could give more info on that cut-out. Any pops, clicks, etc? Do you play it loud?
Btw by your description its easy to deduct that you get highs because the tweeters still operate. The mid and lows aren't driven, thus busted door speakers. I don't have tweeters in mine btw.
You can still check for yourself. A blown speaker is a, well, visually blown speaker. If you can manage to get the panel off the door, which was a headacke, and be careful TO NOT PRY ON THE DOOR LEVER BEZEL because it pops off by itself when you take the whole panel off, check around the cone. Mine had a hole where the membrane was glued to the cone.
Also be careful when you bring it to get detailed. I knew it was going to happen because i had a big stain to get taken care of on the fabric covering the left rear speaker; the shampoo ate the speaker so now im running with one original speaker and some cheap XXX's as replacements in the door.